1. Field of the Invention
This invention relates to a dew-sensing hygroscopic element, and more particularly to a dew-sensing hygroscopic element which has high water-resistance, can be easily manufactured and is difficult to deteriorate.
2. Prior Art
When water is dewed on the surface of a head drum in a video tape recorder (VTR), a running magnetic tape in contact with such surface tends to adhere to the surface of the head drum to be entwined round the head drum, so that the magnetic tape often cannot run in a stable condition. In order to detect the dew water (moisture) on the surface of the head drum, a humidity-sensitive element of an electrical hygrometer is attached to the surface of the head drum in a VTR. The impedance of the humidity-sensitive element changes with changes in humidity. When the humidity is above a predetermined level, the electric hygrometer signals an alarm.
Humidity-sensitive materials for electrical hygrometers are generally classified into semiconductive materials and hygroscopic high molecular weight materials.
Electrical hygrometers using hygroscopic high molecular weight materials are generally divided into two types. In the electrical hygrometers of one type, electrically-conductive particles are dispersed into hygroscopic high molecular weight materials. The hygroscopic high molecular weight materials swell with humidity and thereby change the distances between the electrically-conductive particles and thus change the electrical resistance thereof. In the electrical hygrometers of the other type, hygroscopic high molecular weight materials contain dissociation ions, and changes in electrical conductivity with changes in humidity occur so that humidity is detected by a change in the generated signal.
The following hygroscopic high molecular weight materials are disclosed as humidity-sensitive materials for electrical hygrometers:
A mixture of silver powder and resins of polyvinyl ester derivatives, such as polyvinyl butyral (Japanese Patent Publication (after acceptance) No. 29627/1968);
Graft-polymerides of acrylic acid chloride and polyamino-polyethylene glycol (Japanese Patent Publication (after acceptance) No. 18836/1970);
A film of metal oxide and carbon powder impregnated with polyvinyl acetate or cellulose acetate (Japanese Patent Publication (after acceptance) No. 27438/1972);
A mixture of carbon powder, polyoxyethylene and cellulose (Japanese Patent Publication (after acceptance) No. 38190/1974);
A mixture of calcium sulfate as a base material, and an inorganic salt and an aqueous solution containing high molecular weight compounds such as PVA (polyvinyl alcohol), as a binding agent (Japanese Patent Publication (after acceptance) No. 20275/1975);
A mixture of a chlorine-containing resin and a polyamide resin (Japanese Patent Publications (no examination) No. 82877/1973; No. 67684/1974; No. 67685/1974 and No. 20781/1975);
Sulfonated cross-linked polystyrene, coated with an inorganic salt solution (Japanese Patent Publications (no examination) No. 17780/1973 and No. 17781/1973); and
A cross-linked film of hydrophilic resins, such as acrylic esters, methacrylate esters, polyvinyl alcohols, polyvinyl acetals, polyacrylamides, polyvinyl pyrrolidones, polyethylene oxides, polyethylene imines, hydroxyethyl cellulose or cellophane (Japanese Patent Publications (no examination) No. 134198/1975).
The hygroscopic high molecular weight compounds of the type into which electrically-conductive particles are dispersed, are disadvantageous in that they exhibit hysteresis and that the humidity indication thereof are unstable in high humidity, such as over a relative humidity of 90%.
On the other hand, the hygroscopic high molecular weight compounds of the other type containing dissociation ions therein exhibit little hysteresis and are more sensitive to humidity. For example, an electrical hygrometer is known in which 0.5 to 1% by weight of LiCl is added to PVA (polyvinyl alcohol) having a saponification equivalent of 80 to 90 (Japanese magazine "Koso Kisho", No. 3, May, 1954), and a Dunmore-type electrical hygrometer is known in which 0 to 2.2% by weight of LiCl is added to PVA having a saponification equivalent of 36 ("Journal of Research of the National Bureau of Standards" vol 123, page 701, 1939). In such electrical hygrometers, the electrical resistance-humidity response is substantially linear and changes in humidity are detected by changes in electric resistance.
However, in the above-described electrical hygrometer, the concentration of LiCl, as an inorganic electrolyte, has a great influence on the linearity of the electrical resistance-humidity response. The concentration of LiCl must be finely adjusted within a relatively narrow range. However, LiCl tends to migrate from the humidity-sensitive element. This fact results in a decrease of the desired concentration of LiCl, and therefore to an increase in the resistance of the humidity-sensitive element. The above-described electrical hygrometers do not adequately function in the presence of dew (moisture) for a normal electrical hygrometer. In these conventional electrical hygrometers, the humidity-sensitive film (PVA) tends to dissolve into water or to become fluid or to deform, at a relative humidity around 100%, namely, when water is dewed or precipitated on the humidity-sensitive film. In such instance, LiCl is apt to dissipate into the precipitated moisture. Accordingly, hygrometers using PVA containing LiCl lack resistance to water, so that they cannot be used as a dew-sensing element. In the conventional electrical hygrometer, water dew is positively prevented from adhering to the humidity-sensitive element.
In order to obtain a hygroscopic element resistive to water dew (i.e. liquid) which can be used as a dew-sensing element, it has been proposed that PVA-ammonium bichromate having a saponification equivalent of 100 be photo-cross-linked (photo-hardened) so as to become water-insoluble (Japanese Patent Publication (after acceptance) No. 20276/1975). In addition, polyvinyl cinnamic acid-PVA copolymers and vinyl acetate-ethylene copolymers are suggested as water-insoluble photosensitivity resins, in place of the above PVA-bichromate. However, a hygroscopic element formed from such polymerized materials has the disadvantages that it is complicated in manufacture, that the diffusion speed of water within such material is low, that the response time of such a material is long, that it is costly and that the Cr.sup.6+ ion derived from ammonium bichromate gives rise to pollution.